HMO2, I'm supposed to get a fiver a week for 2 kids, so it'll probably the same for you (I think that's the minimum sum), but Gingerbread are wrong - you are allowed to keep the first £5 of maintenance without it affecting your IS. It's only a fiver, but when you're on IS, that fiver can be important.
There's nothing immature about expecting your child's father to give some level of support (even if very low) to his own flesh and blood. Go for it, your children need the money.